Volume 8, Issue 5 (May 2011)
Hydrogen Absorption Mechanism of Zirconium Alloys Based on Characterization of Oxide Layer
In order to get a better understanding of the mechanism governing hydrogen absorption behavior in Zr-based alloys, various characterization techniques were applied to the oxide layers of three alloys: Zry-2, GNF-Ziron (Zry-2-based alloy with ∼0.26 wt % Fe), and VB (Zr-based alloy containing ∼0.5 wt % Sn, ∼0.5 wt % Fe, and ∼1 wt % Cr). Out-of-pile corrosion tests were carried out in 400 °C steam and 290 °C LiOH water. For both tests, the hydrogen absorption decreased with higher iron content in the alloys, in the order of Zry-2>GNF-Ziron>VB, despite different kinetics of a parabolic law in the former test and a linear law in the latter test. The acceleration of hydrogen absorption in the LiOH water was ascribed to the formation of degraded or open grain boundaries up to locations very near the metal/oxide interface. The pre-transition steam oxides of 1.4–1.7 μm had a double layer structure composed of the outside non-protective oxide of monoclinic ZrO2 with faster diffusivity and the inside barrier layer of predominantly tetragonal ZrO2 with slower diffusivity. The thickness of the barrier layer of about 0.8–0.9 μm was not changed for the different alloys. The diffusion coefficient of deuterium in the VB oxide was approximately half of that in the GNF-Ziron oxide. This factor for the diffusivity was consistent with their hydrogen pickup performance. The higher compressive stress in the barrier layer was directly linked to the higher hydrogen pickup resistance of the alloys. Preferential dissolution of alloy elements from the second-phase particles (SPPs) into the oxide matrix was evinced for iron, and was very limited for chromium and nickel. These two elements had a tendency to exist as precipitates in the oxide layers, chromium mainly as oxide, and nickel mainly as metal. The superior hydrogen absorption performance of VB containing higher iron content and the SPPs with larger size and number density was attributable to the dissolved iron effect and higher compressive stress state generated from the delayed oxidation of the SPPs in the barrier layer.